1. Field of the Invention
The invention relates generally to mechanical or electrostatic clamping chucks for holding a workpiece and, more specifically, to an improved topographical structure of a support surface of such chucks to increase heat transfer gas distribution along the bottom surface of a workpiece retained by the chuck.
2. Description of the Background Art
Mechanical and electrostatic clamping chucks are used for holding a workpiece in various applications ranging from holding a sheet of paper in a computer graphics plotter to holding a semiconductor wafer within a semiconductor wafer process chamber. In semiconductor wafer processing equipment, mechanical and electrostatic chucks are used for clamping wafers to a pedestal during processing. The pedestal may form both an electrode (in electrostatic chuck applications) and a heat sink. These chucks find use in etching, chemical vapor deposition (CVD), and physical vapor deposition (PVD) applications.
Mechanical chucks typically secure a workpiece to the chuck by applying a physical holding force to a clamping ring or calipers located at the periphery of the workpiece. The workpiece is held in place until the physical force is reversed and the clamping ring or calipers retract. Electrostatic chucks perform this task by creating an electrostatic attractive force between the workpiece and the chuck. A voltage is applied to one or more electrodes in the chuck so as to induce opposite polarity charges in the workpiece and electrodes, respectively. The opposite charges pull the workpiece against the chuck, thereby retaining the workpiece. More specifically, in a "unipolar" electrostatic chuck, voltage is applied to the conductive pedestal relative to some internal chamber ground reference. Electrostatic force is established between the wafer being clamped and the electrostatic chuck. When the voltage is applied, the wafer is referred back to the same ground reference as the voltage source by a conductive connection to the wafer. Alternatively, a plasma generated proximate the wafer can reference the wafer to ground, although some voltage drop occurs across plasma sheaths that form at both the wafer being clamped and the reference electrode.
The materials and processes used to process a wafer are extremely temperature sensitive. Should these materials be exposed to excessive temperature fluctuations due to poor heat transfer from the wafer during processing, performance of the wafer processing system may be compromised resulting in wafer damage. To optimally transfer heat between the wafer and a chuck, a very large electrostatic or physical force is used in an attempt to cause the greatest amount of wafer surface to physically contact a support surface of the chuck. However, due to surface roughness of both the wafer and the chuck, small interstitial spaces remain between the chuck and wafer that interfere with optimal heat transfer.
To achieve further cooling of the wafer during processing, an inert gas such as Helium is pumped into the interstitial spaces formed between the wafer and the support surface. This gas acts as a thermal transfer medium from the wafer to the chuck that has better heat transfer characteristics than the vacuum it replaces. The chucks are generally designed to prevent the heat transfer gas from escaping into the surrounding low pressure atmosphere (i.e., the reaction chamber). Specifically, the support surface of electrostatic chucks have a circumferential raised rim having a diameter that is approximately equal to the diameter of the wafer and a flex circuit covering the support surface of the underlying pedestal. The flex circuit is usually a conductive material encased in a flexible dielectric material. The conductive material is patterned to form the electrostatic electrode(s). The dielectric material insulates the conductive material from other conductive components and also acts as a gasket. Once the wafer is clamped, a gas tight seal is created between the wafer and the rim. As such, heat transfer gas leakage from beneath the wafer at the rim is minimized. The clamping ring of mechanical chucks pushes down against a lip seal at the edge of the support surface of its pedestal to eliminate leakage. The lip seal is smaller than the dielectric "gasket" used in the electrostatic chuck, but is similar in principle. To further enhance the cooling process, the chuck is typically water-cooled via conduits within the pedestal. This cooling technique is known as backside gas cooling.
In the prior art, heat transfer gas distribution to the interstitial spaces is osmotic. Once a certain gas pressure is reached, pumping ceases and the gas becomes stagnant under the wafer. Since some of the interstitial spaces may not be interconnected, some spaces do not receive any heat transfer gas. This condition can lead to a non-uniform temperature profile across the wafer during processing and result in wafer damage. Effective and uniform heat conduction away from the wafer is an important aspect of the manufacturing process. Therefore, maximizing wafer area either in contact with the support surface or exposed to the heat transfer gas should contribute to the greatest heat transfer rate. As such, backside gas cooling art developed based on this premise.
However, the physical limitations of existing technology do not provide the necessary conditions for a uniform distribution of heat transfer gas in all of the interstitial spaces beneath the wafer. Existing pedestal topographies limit the effectiveness of the heat transfer process because they have a generally flat support surface and the wafers have a generally flat bottom surface. Ideally, these flat surfaces would have no defects or deviations so that the entire bottom surface of the wafer would contact the support surface to allow for maximum heat transfer from the wafer to the pedestal. General topographical anomalies create a condition where not all of the wafer is in contact with this support surface.
Electrostatic chucks attempt to solve this problem by providing a more uniform heat transfer gas layer across the entire bottom surface of the wafer. The flex circuit may be shaped in various configurations such as a flat plate across most of the support surface, as a series of concentric rings or radial arms to disperse the heat transfer gas across the entire bottom surface of the wafer. Additionally, the flex circuits have grooves or channels so the heat transfer gas can flow through and across the flex circuit somewhat uniformly. Nonetheless, the aforementioned interstitial spaces containing no gas still form and distribution of the gas could be compromised. Mechanical chucks may suffer from premature breakdown of the clamping components or non-uniform compression of the clamping ring or calipers which contributes to a loss of heat transfer gas pressure at the periphery lip seal.
Some gas leakage is expected in any type of chuck; therefore, a minimum gas pressure is maintained under the wafer at any given time to assure adequate gas density. If the gas escapes at a rate faster than anticipated, heat transfer characteristics of the device again become unstable and unreliable. Additionally, wafers secured to both types of chucks tend to bow at their centers resulting from the heat transfer gas pressure building up on the underside of the wafer. For mechanical chucks, the bowing action also could potentially lift the wafer away from the sealed areas. This condition contributes to a non-uniform heat transfer gas condition which results in poor temperature control across the wafer underside. Consequently, during processing, the temperature non-uniformity may result in non-uniform processing and wafer damage.
Therefore, there is a need in the art for an improved topographical structure of a semiconductor wafer processing chuck that maximizes heat transfer rate and improves temperature control and uniformity across the wafer without adding considerably to manufacturing costs.